For example, Patent Document 1 discloses a frame structure which relates to a frame structure of a seat back part and in which, without using pipes, plate-shaped members given increased strength by closing their cross sections are used and are arranged so that their width direction becomes a front and rear direction of a vehicle. In this case, as the frame structure, frames made of the four separate plate-shaped members corresponding to a pair of left and right side frames, an upper frame, and a lower frame are prepared in advance, and these frames are welded with end portions thereof confronted with each other, to be integrated.